1. Field of the Invention
The present invention relates to the field of integrated circuit testing devices, and more specifically, to a probe device for testing integrated circuit wafers.
2. Prior Art
In the modern manufacture of integrated circuits, several hundred to several thousand integrated circuits are fabricated in a single circuit wafer. In such a batch fabrication process, the yield of usable integrated circuits is very low. Due to this low yield, a method has been developed to test each individual wafer to determine the functional integrated circuits contained therein. Without such a method, each integrated circuit must be severed from the wafer and mounted so that it may be tested individually.
Presently, probe cards, in combination with automatic test equipment, are being utilized during the manufacture of integrated circuits to determine the usability of the individual circuits. In general, the probe cards consist of multi-probe members which are mechanically held in contact with the circuit wafers. Such mechanical contact allows electrical testing of the individual integrated circuits prior to their severance from the wafer. Thus, the required input voltages and input signals may be provided to the individual integrated circuit by the automatic test equipment and the resulting output signals may be monitored and evaluated by the test equipment. Often, it is desired to use these probe cards to couple high frequency signals to the integrated circuit wafers and to allow monitoring of high frequency output signals. Also, it would be valuable to use these probe cards at elevated temperatures to determine if the integrated circuits are functional at such temperatures.
One prior art attempt to provide the testing capabilities noted above is what is generally referred to as the blade probe card. The blade probe card consists of an epoxy-glass printed circuit card to which is affixed a plurality of berylium-copper blades. A metal needle-like probe is then soldered to each blade and all the probes are configured so that they may contact the pads of an integrated circuit wafer.
The blade probe card, however, has several distinct disadvantages. The greatest disadvantage is its high electrical capacitance between circuit paths resulting from the parallel configuration of the metal blades. That is, the necessary closeness and parallelism of the metal blades results in a capacitive effect between the individual blades. Due to this high capacitance, the blade probe card cannot be used for testing a variety of integrated circuits, including some metal-oxide silicon (MOS) circuits, and for general high frequency testing.
Another disadvantage of the blade probe card is the lack of stiffness of the berylium copper blade. This blade will flex under light pressure and when bent, will only slowly return to its original configuration. The lack of stiffness produces alignment and planarization problems, resulting in a variation of force between the probes when they are applied to the circuit wafer. This variation in force produces two detrimental results. First, those probes which will apply more force to the circuit wafer can cause damage to the corresponding wafer pad. Second, the variation in force results in inconsistent contact resistance from one probe to another. A further problem with the blade probe card is its inability to be used at elevated temperatures since the plastic components of the epoxy-glass card degrade at such temperatures.
A still further disadvantage of the blade probe card results from the low surface resistance and low dielectric constant of the epoxy-glass material. Such probe cards optimally should have infinite surface resistance to allow total isolation of circuit paths. However, since the epoxy-glass material has a low surface resistance, the individual circuit paths are allowed to interact to the detriment of integrated circuit testing. Also, the low dielectric constant of the epoxy-glass material limits the upper frequency at which the card can be used because of the resulting higher capacitance between individual circuit paths.
A second prior art attempt to provide the testing capabilities noted above is the Epoxy-ring card. These cards consist of an Epoxy-glass card which has probes soldered to copper strips which are affixed to the Epoxy-glass. However, the epoxy-ring card also has a variety of disadvantages. One disadvantage, difficulty of repair, stems from the method of affixing the copper strips to the Epoxy-glass. The copper strip is laminated or glued to the Epoxy-board and the probe is then soldered to the strip. If a probe becomes damaged and must be replaced, a new probe will have to be soldered to the copper strip. However, this heating of the strip causes the adhesive which holds it to the card to degrade, allowing the strip to move up and away from the card. Such damage to the copper strip prevents the card from being used again. Repair of the Epoxy-ring card is further hindered by the difficulty of properly aligning the new probe with the original probes.
Another problem associated with the Epoxy-ring card is the flexibility of the card. Since such cards are not rigid, the individual probes soon lose their planarity and alignment with the other probes. Such lack of planarity, as in the case of the blade probe card, results in damage to the circuit wafers and variation in contact resistance.
Further disadvantages of the epoxy-ring card, just as in the blade probe card, are a consequence of the utilization of Epoxy-glass material. The epoxy-ring card also cannot be used at elevated temperatures since the plastic components of the glass material will deteriorate. In addition, the low surface resistance of the glass material and its low dielectric constant allows interaction of the input and monitoring lines as well as limiting its high frequency use.
Due to the diversity of integrated circuits presently available, and the fact that each type of integrated circuit has its own requirements for testing, one piece of test equipment has typically been designed to automatically test each type of circuit. This customized test equipment has required a substantial investment to design and develop, but each is limited by the one class of integrated circuits it can test and even this specialization has not provided the precision testing which is required.
One major limitation in the present automatic test equipment has been the inability to place the necessary compensating networks physically close to the integrated circuit under test. Thus, low level signals which are often at high frequency must travel a great distance from the probe card to the test equipment. Due to the impedance of this long circuit path, degradation of the signal which is to be monitored results so that the testing loses its accuracy. For example, the testing capability of the present automatic test equipment would be greatly improved if a differential amplifier were placed near a CMOS integrated circuit which is being tested. Such an amplifier would significantly improve the signal to noise ratio of the signal which is monitored by the automatic test equipment. Similar improvements could be created by placing an amplifier which has unity gain and matched impedance close to an ECL circuit which is to be tested.
Also, placing circuits near the integrated circuit under test would allow one central piece of automatic test equipment to service more than one class of circuits. That is, only the testing capabilities common to several classes of integrated circuits would be contained in the central test equipment while probe cards which contain the specialized circuits required for each class may be utilized with the central equipment. In this manner, the overall cost of automatic test equipment could be reduced by making one automatic test station capable of testing several classes of circuits. However, the probe cards of the prior art are not capable of allowing placement of such compensating circuits near the circuit under test since neither the proper support nor the necessary isolation from the monitored signal can be provided.
Therefore, what has been needed is a probe card which has low electrical capacitance so that it may be utilized in the testing of integrated circuits at high frequencies, which has probe tips which remain in a planar configuration, which has high electrical isolation between circuit paths, which is easily repaired, which is insensitive to elevated temperature, and which can provide a compensating circuit adjacent each probe tip.